Transport control method, a transport apparatus, and a printing apparatus

ABSTRACT

A transport control method for use in transporting a medium in a predetermined direction by an upstream drive roller and a downstream drive roller, to operate the downstream drive roller by using PID control based on a detection value of a tension sensor disposed downstream of the upstream drive roller and upstream of the downstream drive roller for detecting tension. The transport control method includes a stable state determining step for determining, at time of the operation, whether a difference between the detection value of the tension sensors and a target value is within a stable width to make a stable state which maintains a stability time; and a gain decreasing step for decreasing a gain of the PID control to be less than an initial value when the difference is determined to be in the stable state.

BACKGROUND OF THE INVENTION (1) Field of the Invention

This invention relates to a transport control method, a transport apparatus, and a printing apparatus for transporting an elongate printing medium in a predetermined direction.

(2) Description of the Related Art

Conventionally, a printing apparatus having this type of transport apparatus includes a paper feeder, a printing station, a takeup roller and the transport apparatus (see Japanese Unexamined Patent Publication No. 2014-24266 (FIG. 1), for example).

The above transport apparatus includes a first drive roller, a second drive roller, a third drive roller, and a fourth drive roller. The first drive roller is disposed downstream of the paper feeder, which supplies elongate printing paper, and has a nip roller for feeding the printing paper from the paper feeder. The second drive roller has a nip roller for feeding the printing paper sent by the first drive roller into a printing area directly under the printing station. The third drive roller (also called a heat roller) dries the printing paper printed at the printing station and wound at a large winding angle thereon, and sends the printing paper downstream. The fourth drive roller has a nip roller for sending the printing paper dried by the third drive roller on to the takeup roller. The transport apparatus further includes a first tension sensor disposed downstream of the first drive roller for detecting the tension of the printing paper sent from the first drive roller, a second tension sensor disposed downstream of the second drive roller and upstream of the printing station for detecting the tension of the printing paper in a position upstream of the printing station, and a third tension sensor disposed upstream of the fourth drive roller for detecting the tension in a position upstream of the fourth drive roller.

The transport apparatus of such construction employs a transport control method which, while driving the second drive roller at a constant transport speed, controls transport of the printing paper by operating the other drive rollers. Specifically, the first drive roller is operated so that the first tension sensor will attain a target value. The third drive roller is operated so that the second tension sensor will attain the target value. Further, the fourth drive roller is operated so that the third tension sensor will attain the target value. Note that the fourth drive roller is made to reflect speed variations of the third drive roller operated in accordance with the second tension sensor, thereby to reflect tension variations occurring in the printing area. The above operations are carried out under PID (Proportional Integral Differential) control. Its gain is fixed, for example, to 100% for a period from a starting point to a stopping point of printing paper transport.

However, the conventional example with such construction has the following problem.

With the conventional transport control method, there occurs large variations in the control amount for the first, third and fourth drive motors at a printing speed based on a substantially constant transport speed, which poses a problem of easily causing paper transport irregularities. Such transport irregularities influence a tense condition of the printing paper in the printing area, which is detrimental to print quality. It is therefore desirable to minimize transport irregularities.

So the inventors herein have tried a transport control with the gain in PID control lowered from 100% to 30%. It has been found that such lowering of the parameter to 30% is effective to check variations in the control amount at the printing speed with a substantially constant transport speed. However, the above measure has entailed a different problem that the tension varies extensively at a time of transport speed acceleration from a stopped state to the printing speed. An excessive variation in tension will apply load in the transport direction to the printing paper, thereby causing damage to the printing paper. It is therefore desirable to minimize variations. Thus, it is unrealistic to conduct transport controls by reducing the gain in PID control.

SUMMARY OF THE INVENTION

This invention has been made having regard to the state of the art noted above, and its object is to provide a transport control method, a transport apparatus, and a printing apparatus which can inhibit transport irregularities, while suppressing tension variations at the time of acceleration, by rendering a gain variable according to a stability level of tension control.

The above object is fulfilled, according to this invention, by a transport control method for use in transporting a medium in a predetermined direction by an upstream drive roller disposed in an upstream position in a transport direction of the medium, and a downstream drive roller disposed in a downstream position in the transport direction, to operate the downstream drive roller by using PID control based on a detection value of a tension sensor disposed downstream in the predetermined direction of the upstream drive roller and upstream of the downstream drive roller for detecting tension of the medium, the transport control method comprising a stable state determining step for determining, at time of the operation, whether a difference between the detection value of the tension sensor and a target value is within a stable width to make a stable state which maintains a stability time; and a gain decreasing step for decreasing a gain of the PID control to be less than an initial value when the difference is determined to be in the stable state.

According to this invention, when the stable state determining step determines that the difference between the detection value of the tension sensor and the target value is within a stable width to make a stable state which maintains a stability time, the gain decreasing step decreases the gain of PID control to be less than the initial value. Therefore, when tension control is not stable such as at a time of acceleration, PID control is carried out with the gain at the initial value. Only when tension control is stable, PID control is carried out with sensitivity lowered by a small gain. As a result, when tension control is unstable as at the time of acceleration, for example, the control amount becomes large. The control amount becomes small when tension control is stable as in a constant speed state. Thus, transport irregularities of the medium can be inhibited while inhibiting tension variations at the time of acceleration.

In this invention, it is preferred that an initial value setting step is executed to set the gain to the initial value when the upstream drive roller starts to be driven from a stopped state, and when the upstream drive roller starts to be decelerated toward the stopped state.

The tension is unstable at times of acceleration and deceleration. By performing the control with the initial value instead of a small gain, therefore, the control amount is increased to be able to inhibit tension variations at the times of acceleration and deceleration.

In this invention, it is preferred that a gain increasing step is executed to increase the gain when, after the gain decreasing step, the difference is determined to have deviated from the stable width, or although within the stable width, is in a nonstable state incapable of maintaining the stability time.

The gain is enlarged to increase the control amount at the time of nonstable state. This can stabilize tension even at times of large tension variations.

In this invention, it is preferred that each of the stable state determining step and the gain decreasing step has the stable width in at least two types.

By determining the stable state of tension with two types of stable width, when decreasing the gain, a clearly defined gain adjustment can be made according to tension variations.

In this invention, it is preferred that the gain decreasing step has a minimum gain set as a lower limit in decreasing the gain to be less than the initial value.

Setting the minimum gain as a lower limit can avoid an excessively small gain lowering sensitivity too much, which would cause an inconvenience in tension adjustment.

In this invention, it is preferred that the gain increasing step has a maximum gain set as an upper limit in increasing the gain.

Setting the maximum gain as an upper limit can avoid an excessively large gain raising sensitivity too high, which would cause an inconvenience in tension adjustment.

In this invention, it is preferred that the gain is a proportional gain.

In transport control of an elongate printing medium, the control can be performed well only by adjusting the proportional gain.

In another aspect of this invention, there is provided a transport apparatus for transporting a medium in a predetermined direction, comprising an upstream drive roller disposed in an upstream position in a transport direction of the medium; a downstream drive roller disposed in a downstream position in the transport direction; a tension sensor disposed downstream in the predetermined direction of the upstream drive roller, and upstream of the downstream drive roller, for detecting tension of the medium; a drive control unit for operating the downstream drive roller by using PID control based on a detection value of the tension sensor; a stable state determining unit for determining, at time of the operation, whether a difference between the detection value of the tension sensor and a target value is within a stable width to make a stable state which maintains a stability time; and a gain adjusting unit for decreasing a gain of the PID control to be less than an initial value when the stable state determining unit has determined that the difference is in the stable state.

According to this invention, when the drive control unit operates the downstream drive roller by PID control, the stable state determining unit determines whether or not the difference between the detection value of the tension sensor and the target value is in a stable width to make a stable state which maintains a stability time. When the stable state is established, the gain adjusting unit makes the gain of PID control smaller than an initial value. Therefore, when tension control is not stable such as at a time of acceleration, PID control is carried out with the gain at the initial value. Only when tension control is stable, PID control is carried out with sensitivity lowered by a small gain. As a result, when tension control is unstable as at the time of acceleration, for example, the control amount becomes large. The control amount becomes small when tension control is stable as in a constant speed state. Thus, transport irregularities of the medium can be inhibited while inhibiting tension variations at the time of acceleration.

In a further aspect of this invention, there is provided a printing apparatus for performing printing while transporting an elongate printing medium in a predetermined direction, comprising a printing station for printing on the printing medium in a printing area disposed along a transport path of the printing medium; an upstream drive roller disposed upstream of the printing area; a downstream drive roller disposed downstream of the printing area; a tension sensor disposed downstream in the predetermined direction of the upstream drive roller, and upstream of the printing area, for detecting tension of the printing medium; a drive control unit for operating the downstream drive roller based on a detection value of the tension sensor and by using PID control; a stable state determining unit for determining, at time of the operation, whether a difference between the detection value of the tension sensor and a target value is within a stable width to make a stable state which maintains a stability time; and a gain adjusting unit for decreasing a gain of the PID control to be less than an initial value when the stable state determining unit has determined that the difference is in the stable state.

According to this invention, when the drive control unit operates the downstream drive roller by PID control, the stable state determining unit determines whether or not the difference between the detection value of the tension sensor and the target value is in a stable width to make a stable state which maintains a stability time. When the stable state is established, the gain adjusting unit makes the gain of PID control smaller than an initial value. Therefore, when tension control is not stable such as at a time of acceleration, PID control is carried out with the gain at the initial value. Only when tension control is stable, PID control is carried out with sensitivity lowered by a small gain. As a result, when tension control is unstable as at the time of acceleration, for example, the control amount becomes large. The control amount becomes small when tension control is stable as in a constant speed state. Thus, transport irregularities of the printing medium can be inhibited while inhibiting tension variations at the time of acceleration. As a result, the quality of printing on the printing medium by the printing station can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown.

FIG. 1 is a schematic view showing an entire inkjet printing system having a transport apparatus according to an embodiment;

FIG. 2 is a schematic view showing a control relationship of a first to a fourth drive rollers;

FIG. 3 is a graph showing a relationship between a command value to the second drive roller, gain, and tension as matched with transport distances of web paper WP;

FIG. 4 is a flow chart showing an example of control;

FIG. 5 is a flow chart showing the example of control;

FIG. 6 is a graph showing variations in tension in the inkjet printing system according to the embodiment;

FIG. 7 is a graph showing variations in command value in the inkjet printing system according to the embodiment;

FIG. 8 is a graph showing variations in tension with a gain set to 100% in an inkjet printing system according to a conventional example;

FIG. 9 is a graph showing variations in command value with the gain set to 100% in the inkjet printing system according to the conventional example;

FIG. 10 is a graph showing variations in tension with the gain set to 30% in the inkjet printing system according to the conventional example; and

FIG. 11 is a graph showing variations in command value with the gain set to 30% in the inkjet printing system according to the conventional example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of this invention will be described hereinafter with reference to the drawings.

FIG. 1 is a schematic view showing an entire inkjet printing system having a transport apparatus according to this embodiment.

An inkjet printing system 1 according to this embodiment includes an inkjet printing apparatus 3, a paper feeder 5 and a takeup roller 7.

The inkjet printing apparatus 3 performs printing on elongate web paper WP. The paper feeder 5 holds a roll of web paper WP to be rotatable about a horizontal axis, and unwinds the web paper WP from the roll of web paper WP to feed it to the inkjet printing apparatus 3. The takeup roller 7 winds up the web paper WP printed by the inkjet printing apparatus 3 about a horizontal axis. Regarding the side from which the web paper WP is fed as upstream and the side to which the web paper WP is discharged as downstream, the paper feeder 5 is disposed upstream of the inkjet printing apparatus 3 while the takeup roller 7 is disposed downstream of the inkjet printing apparatus 3.

The inkjet printing apparatus 3 corresponds to the “printing apparatus” in this invention. The web paper WP corresponds to the “printing medium” and the “medium” in this invention.

The inkjet printing apparatus 3 includes a first drive roller M1 in an upstream position thereof for taking in the web paper WP from the paper feeder 5. The web paper WP unwound from the paper feeder 5 by the first drive roller M1 is transported downstream toward the takeup roller 7 along a plurality of rotatable transport rollers 11.

An edge position controller 15 is disposed downstream of the first drive roller M1. When the web paper WP wanders off in directions perpendicular to a transport direction, the edge position controller 15 will automatically adjust and control the web paper WP to be transported to a right position.

A second drive roller M2 is disposed downstream of the edge position controller 15. The web paper WP fed downstream by the second drive roller M2 has the transport direction changed by a transport roller 11 disposed downstream of the second drive roller M2, to advance along a transport path to a printing area PA where printing is done. This transport roller 11 has a rotary encoder 13 mounted thereon. The printing area PA has a plurality of transport rollers 11 arranged along the transport path of the web paper WP.

A printing station 19 is disposed above the printing area PA. The printing station 19 in this embodiment includes four inkjet heads 19 a-19 d, for example. The inkjet head 19 a in the most upstream position, for example, dispenses ink droplets of black (K), the next inkjet head 19 b ink droplets of cyan (C), the next inkjet head 19 c ink droplets of magenta (M), and the next inkjet head 19 d ink droplets of yellow (Y). The inkjet heads 19 a-19 d are arranged separately at predetermined intervals in the transport direction.

The web paper WP printed in the printing area PA has the transport direction changed by a downstream transport roller 11. A third drive roller M3 is disposed ahead. The third drive roller M3 winds the web paper WP at a large winding angle, and contacts the web paper WP to dry the ink droplets on the web paper WP. This third drive roller M3 has a built-in heater, and is also called a heat drum.

The web paper WP dried by the third drive roller M3 is sent by a fourth drive roller M4 to the takeup roller 7, while having its direction changed by a plurality of transport rollers 11. An inspecting unit 23 is disposed upstream of the fourth drive roller M4. The inspecting unit 23 inspects the web paper WP printed at the printing station 19. The takeup roller 7 takes up in a roll form the web paper WP inspected by the inspecting unit 23.

The first drive roller M1, second drive roller M2 and fourth roller M4 described above individually have nip rollers 25 rotatably attached. A carrying force to the web paper WP is applied by the nip roller 25 pinching the web paper WP with each drive roller. The pressing force of each nip roller 25 is applied by an air cylinder (not shown), for example. The nip rollers 25 are formed of an elastic material such as rubber, for example.

A first tension sensor TP1 is disposed downstream of the first drive roller M1 and upstream of the edge position controller 15. A second tension sensor TP2 is disposed downstream of the second drive roller M2 and upstream of the printing area PA. A third tension sensor TP3 is disposed downstream of the third drive roller M3 and upstream of the fourth drive roller M4. The first to third tension sensors TP1-TP3 successively detect current tension applied to the web paper WP, and output detection values of the tension.

The inkjet printing apparatus 3, paper feeder 5 and takeup roller 7 are operable under overall control of a main controller 49.

The main controller 49 includes a control unit 51 and a storage unit 57. The control unit 51 is constructed of a CPU and other components. The control unit 51, upon instructions by the operator to start printing, controls transportation of the web paper WP by giving the second drive roller M2 a command value indicating a transport speed, and giving the first drive motor M1, third drive motor M3 and fourth drive motor M4 control amounts based on PID control as described in detail hereinafter. The control is carried out with reference to the command value given to the second drive roller M2, to realize a printing speed provided by the transport speed at the time of printing which meets printing conditions set beforehand by the operator. The control unit 51 determines the transport speed and transport distances of the web paper WP based on output signals of the rotary encoder 13. The printing conditions are conditions relating to print quality, such as the transport speed of the web paper WP and each target value of tension in each part applied to the web paper WP, for example. The storage unit 57 stores beforehand two types of stable width, a stability time, a target value of tension, an initial value of gain, a decrease amount of gain, an increase amount of gain, two types of minimum gain, and a maximum gain to be described hereinafter.

Reference is now made to FIG. 2. FIG. 2 is a schematic view showing a control relationship of the first to fourth drive rollers.

The control unit 51 carries out PID control based on a difference between a target value of tension to be applied to the web paper W in the location of the first tension sensor TP1 and a detection value of the first tension sensor TP1, and gives a control amount to the first drive roller M1 to make the detection value equal to the target value. The control unit 51 carries out PID control based on a difference between a target value of tension to be applied to the web paper W in the location of the second tension sensor TP2 and a detection value of the second tension sensor TP2, and gives a control amount to the third drive roller M3 to make the detection value equal to the target value. Similarly, the control unit 51 carries out PID control based on a difference between a target value of tension to be applied to the web paper WP in the location of the third tension sensor TP3 and a detection value of the third tension sensor TP3, and gives a control amount to the fourth drive roller M4 to make the detection value equal to the target value. Further, the control unit 51 preferably performs control by adding to the above control amount for the fourth drive roller M4 an adjustment value based on an amount of change in the rotating speed of the third drive roller M3.

The second drive roller M2 described above corresponds to the “upstream drive roller” in this invention. The third drive roller M3 corresponds to the “downstream drive roller” in this invention. The second tension sensor TP2 described above corresponds to the “first tension sensor” in this invention.

The above control unit 51 carries out PID control based on differences between detection values and target values, and for this purpose the initial value is set to 100%. In this embodiment, control is carried out based on only PI of PID control, and the gain adjusted when the state is determined stable is only proportional gain (P) as described hereinafter.

The control unit 51 described above corresponds to the “drive control unit”, “stable state determining unit” and “gain adjusting unit” in this invention.

Reference is now made to FIG. 3. FIG. 3 is a graph showing a relationship between the command value to the second drive roller M2, gain and tension as matched with transport distance of the web paper WP.

The control of the command value to the second drive roller M2, namely the transport speed used as the reference, is a control for making a converted speed based on the above rotary encoder 13 constant. The command value to the second drive roller M2 is therefore zero for distance zero (which corresponds also to time), which command value provides printing speed SP at distance d1. The command value given begins deceleration from the printing speed SP at distance d18 where the printing ends, and reduces the transport speed to zero at distance d19.

On the other hand, the control amounts to the first drive roller M1, third drive roller M3, and fourth drive roller M4 vary as shown in a vertically oscillating dotted line in the graph of the command value to the second drive roller M2, for example. Specifically, the control unit 51 outputs the command value according to a difference between each of the detection values of tension by the first to third tension sensors TP1-TP3 and the target value, and the proportional gain. When the control unit 51 determines the differences, it is preferred to make the detection values of tension into moving average deviations. This can suppress disturbance of the control caused by temporary variations of tension due to the influence of noise or external factors.

Adjustment of the proportional gain by the control unit 51 noted above will be described. Here, the storage unit 57 is assumed to have stored therein beforehand: stable width A=±500 g, stable width B=±1000 g, stability time ST=5 seconds, initial value of gain=100%, decrease amount of gain G1=20%, increase amount of gain G2=20%, minimum gain Gmin1=20%, minimum gain Gmin2=60%, and maximum gain Gmax=100%. These values such as of stable width A are given only by way of example, and can be set variously according to objects to be transported, transport routes, characteristics of the rollers, ambient environment of the apparatus, and so on.

Stable width A and stable width B provide ranges of variation of tension for determining whether or not the tension is in a stable state. Tension is determined to be in the stable state when difference Δts between a detection value and a target value of tension is within these stable widths A and B and this state maintains the next stability time ST. Stability time ST=5 seconds provides a time for determining whether or not difference Δts is in the stable state. The initial value of gain provides a gain at the time PID control is started, a gain at the time of increasing transport speed from stopped state to printing speed, and a gain the time of stopping transport speed from printing speed. The decrease amount of gain G1 provides an amount of gain subtracted from a current gain when stable state is determined. The increase amount of gain G2 provides an amount of gain added to a current gain when a nonstable state is determined. Minimum gain Gmin1 provides a lower limit to a gain resulting from a subtraction made when a stable state is determined in stable width A. Minimum gain Gmin2 provides a lower limit to a gain resulting from a subtraction made when a stable state is determined in stable width B. Maximum gain Gmax provides an upper limit to a gain resulting from an addition.

In the above example, only the same decrease amount of gain G1 is subtracted when a stable state is determined, whether the determination is based on the stable width A or the determination is based on the stable width B. However, since the stable state determined based on the stable width A is higher in stability, the gain may be decreased by a decrease amount of gain G2 having a larger value than the decrease amount of gain G1 when the stable state is determined based on stable width A. Since the gain can thereby be made small quickly, tension control can be performed with increased stability when the stable state is continued.

In the following description, the tension and gain in FIG. 3 may represent any combination of the first tension sensor TP1 and first drive roller M1, the second tension sensor TP2 and third drive roller M3, or the third tension sensor TP3 and fourth drive roller M4.

In FIG. 3, from distance 0 to distance d3 the gain is maintained at the initial value which is 100%. Assume here that, in the stability time ST from distance d2 to distance d3, difference Δts between target value TG of tension and the detection value of tension does not fit in the stable width A, but fits in the stable width B. The tension is therefore determined to be in a stable state. So the decrease amount of gain G1 (=20%) which is a decrease value at the time of stable width B is subtracted from the current gain (=100%), to decrease the gain to 80%. Assume that the tension in the stability time ST from distance d4 to distance d5 fits in the stable width B. Since the tension is therefore determined to be in a stable state, the decrease amount of gain G1 (=20%) is subtracted from the current gain (=80%), to decrease the gain to 60%.

Assume that, in the stability time ST from distance d6 to distance d7, difference Δts fits in the stable width A smaller than the stable width B. Since the tension is therefore determined to be in a stable state, the decrease amount of gain G1 (=20%) which is the decrease value at the time of stable width A is subtracted from the current gain (=60%), to decrease the gain to 40%. Assume that, in the stability time ST from distance d8 to distance d9, difference Δts fits in the stable width A. Then, the decrease amount of gain G1 (=20%) is subtracted from the current gain (=40%), to decrease the gain to 20%. Here, even if the stable width A can be further maintained for another stability time ST, the gain will not be further reduced since restriction is set by the lower limit of minimum gain Gmin1=20%. This can avoid the inconvenience of PID control being destabilized by an excessively small gain.

Assume that, in the stability time ST from distance d10 to distance d11, difference Δts has deviated from the stable width A and has further shifted to the stable width B. In this case, whether the stability time ST is maintained is not questioned. It is adequate to check whether or not difference Δts of tension exceeded stable width A. That is, it is determined whether the control has begun to be unstable. Since the tension is therefore determined to be in a nonstable state, the gain increase amount G2 (=20%) is added to the current gain (=20%), to increase the gain to 40%. Assume that, in the stability time ST from distance d12 to distance d13, tension is outside the stable width B. Since the tension is therefore determined to be in a nonstable state, the gain increase amount G1 (=20%) is added to the current gain (=40%), to increase the gain to 60%. Assume that, in the stability time ST from distance d15 to distance d16, difference Δts fits in the stable width A. Since the tension is therefore determined to be in a stable state, the decrease amount of gain G1 (=20%) which is the decrease value at the time of stable width A is subtracted from the current gain (=60%), to decrease the gain to 40%. Assume that, in the stability time ST from distance d16 to distance d17, difference Δts fits in the stable width B. Since the tension is therefore determined to be in a stable state, the decrease amount of gain G1 (=20%) is subtracted from the current gain (=40%), to decrease the gain to 20%.

When the control unit 51 finishes the printing and shifts the transport speed from printing speed to stopped state, maximum gain Gmax=100% is set irrespective of the amount of a current gain.

The second drive roller M2, third drive roller M3, second tension sensor TP2, and control unit 51 correspond to the “transport apparatus” in this invention.

Next, a control flow by the control unit 51 will be described with reference to FIGS. 4 and 5. FIGS. 4 and 5 are a flow chart showing an example of control.

Step S1

The control unit 51 sets the gain to the initial value (=100%). The initial value need not necessarily be 100%, but should preferably be a maximum value of gains to be adjusted.

Step S2

The control unit 51 repeatedly checks whether acceleration is completed, and moves to the next process in step S3 upon completion of acceleration.

Step S3

The control unit 51 branches the process based on whether deceleration is started or not. When deceleration is started, the gain is set to the initial value (=100%).

Step S5

Difference Δts between the detection value of tension and the target value is calculated. The detection values of tension at the time of calculating difference Δts, preferably, are moving average deviations for the reason noted hereinbefore.

Steps S6-S9 carry out processes for lowering the gain when difference Δts of tension is within the stable width A to maintain a stability time ST.

Step S6

Checking is made whether or not difference Δts is within the stable width A and this state maintains the stability time ST, and the process branches according to the result. When difference Δts is within the stable width A to make a stable state which maintains the stability time ST, the process moves to step S7. Otherwise it is a nonstable state, and the process moves to step S2.

Step S7

When step S6 indicates a stable state in which difference Δts is within the stable width A and this state maintains the stability time ST, the gain decrease amount G1 is subtracted from a current gain to make a new gain.

Step S8

The gain resulting from the subtraction is compared with the minimum gain Gmin1, and the process branches according to a result of the comparison. When the new gain is larger than minimum gain Gmin1, the process branches to step S2. When the new gain is smaller than minimum gain Gmin1, the process moves to step S9.

Step S9

When the gain resulting from the subtraction is smaller than minimum gain Gmin1, the gain is set to minimum gain Gmin1 and the gain is fixed irrespective of the arithmetic result. This can avoid an excessively small gain lowering sensitivity too much, which would cause an inconvenience in tension adjustment.

Steps S10-S13 carry out processes for lowering the gain when difference Δts of tension is within the stable width B to maintain a stability time ST. A gain decrease amount G1 a larger than the gain decrease amount G1 may be used to increase the decrease amount in the case of stable width A.

Step S10

Checking is made whether or not difference Δts is within the stable width B and this state maintains the stability time ST, and the process branches according to the result. When difference Δts is within the stable width B to make a stable state which maintains the stability time ST, the process moves to step S11. Otherwise, the process moves to step S14.

Step S11

When step S10 indicates a stable state in which difference Δts is within the stable width B and this state maintains the stability time ST, the gain decrease amount G1 is subtracted from a current gain to make a new gain.

Step S12

The gain resulting from the subtraction is compared with the minimum gain Gmin2, and the process branches according to a result of the comparison. When the new gain is larger than minimum gain Gmin2, the process branches to step S2. When the new gain is smaller than minimum gain Gmin2, the process moves to step S13.

Step S13

When the gain resulting from the subtraction is smaller than minimum gain Gmin2, the gain is set to minimum gain Gmin2 and the gain is fixed irrespective of the arithmetic result. This can avoid an excessively small gain lowering sensitivity too much, which would cause an inconvenience in tension adjustment. The reason for minimum gain Gmin2>minimum gain Gmin1 is that the stable width B is wider than the stable width A and has larger tension variations than the stable width A, and therefore preferably has a larger gain than in the stable state within the stable width A.

Steps S14-S17 confirm a shift of difference Δts from stable width A to stable width B. In other words, these steps check whether or not the tension control has been disturbed.

Step S14

Checking is made whether or not difference Δts has shifted from stable width A to stable width B or has shifted outside stable width B, and the process branches according to the result. When it has shifted, the process moves to step S15. Otherwise the process moves to step S18.

Step S15

The gain increase amount G2 is added to the current gain.

Step S16

The gain resulting from the addition is compared with the maximum gain Gmax, and the process branches according to a result of the comparison. When the new gain is larger than the minimum gain Gmax, the process branches to step S17. When the new gain does not exceed the maximum gain Gmax, the process moves to step S2.

Step S17

The gain is fixed to the maximum gain Gmax. Setting the maximum gain Gmax as an upper limit can avoid an excessively large gain raising sensitivity too much, which would cause an inconvenience in tension adjustment.

Steps S18-S21 confirm a shift of difference Δts beyond stable width B. In other words, these steps check whether or not the tension control has been further disturbed.

Step S18

Checking is made whether or not difference Δts has shifted beyond the stable width B and this state maintains stability time ST, and the process branches according to the result. When difference Δts is outside the stable width B to make a nonstable state which maintains the stability time ST, the process moves to step S19. Otherwise the process moves to step S2.

Step S19

The gain increase amount G2 is added to the current gain.

Step S20

The gain resulting from the addition is compared with the maximum gain Gmax, and the process branches according to a result of the comparison. When the new gain is larger than the minimum gain Gmax, the process branches to step S21. When the new gain does not exceed the maximum gain Gmax, the process moves to step S2.

Step S21

The gain is fixed to the maximum gain Gmax.

According to this embodiment, the control unit 51, when operating the first drive roller M1, third drive roller M3, and fourth drive roller M4 by PID control, determines whether or not differences Δts between the detection values of the first-third tension sensors TP1-TP3 and the target value are in the stable width A or B to make a stable state which maintains a stability time ST. When the stable state is established, the gain of PID control is made smaller than an initial value. Therefore, when tension control is not stable such as at a time of acceleration, PID control is carried out with the gain at the initial value. Only when tension control is stable, PID control is carried out with sensitivity lowered by a small gain. As a result, when tension control is unstable as at the time of acceleration, for example, the control amount becomes large.

The control amount becomes small when tension control is stable as in a constant speed state. Thus, transport irregularities of the web paper WP can be inhibited while inhibiting tension variations at the time of acceleration. As a result, the quality of printing on the web paper WP by the printing station 19 can be improved.

The gain is set to the initial value when starting drive from a stopped state to the printing speed and when starting deceleration from the printing speed to a stopped state. This can inhibit tension variations at the times of acceleration and deceleration. Further, the gain is enlarged to increase the control amount at the time of nonstable state. This can stabilize tension even at times of large tension variations.

A comparison is now made between the foregoing embodiment and a conventional example with reference to FIGS. 6 to 11.

FIG. 6 is a graph showing variations in tension in the inkjet printing system according to the embodiment. FIG. 7 is a graph showing variations in the command value in the inkjet printing system according to the embodiment. FIG. 8 is a graph showing variations in tension with a gain set to 100% in an inkjet printing system according to the conventional example. FIG. 9 is a graph showing variations in the command value with the gain set to 100% in the inkjet printing system according to the conventional example. FIG. 10 is a graph showing variations in tension with the gain set to 30% in the inkjet printing system according to the conventional example. FIG. 11 is a graph showing variations in the command value with the gain set to 30% in the inkjet printing system according to the conventional example.

In this embodiment, as shown in the area enclosed by a dotted line in FIG. 6, variations in tension are inhibited also in an acceleration process in which transport speed is increased from a stopped state to the printing speed. As shown in the area enclosed by a dotted line in FIG. 7, it will be seen that variations in the control amount particularly to the third drive roller M3 are inhibited to inhibit transport irregularities.

In the conventional example having the gain set to 100%, on the other hand, there occur little tension variations as shown in FIG. 8. However, as shown in the area enclosed by a dotted line in FIG. 9, in particular, it will be seen that variations in the control amount to the third drive roller M3 are large.

In the conventional example having the gain set to 30%, as shown in the area enclosed by a dotted line in FIG. 11, variations in the control amount to the third drive roller M3 are inhibited. However, as shown in the area enclosed by a dotted line in FIG. 10, it will be seen that tension variations are large in the acceleration process in which transport speed is increased from stopped state to printing speed.

These graphs of tension, command value, and control amount show that this embodiment produces advantageous effects compared with the conventional example.

This invention is not limited to the foregoing embodiment, but may be modified as follows:

(1) In the foregoing embodiment, the gain is reduced a plurality of times when a stable state is indicated. When a stable state is indicated once, the gain may be reduced only once.

(2) The foregoing embodiment provides two types of stable width. This invention is not limited to this. For example, the stable width may be provided in one type, or in three or more types.

(3) In the foregoing embodiment, only the proportional gain is adjusted. This invention is not limited to this. For example, depending on what is transported, integral gain (I) and/or differential gain (D) may be adjusted besides the proportional gain (P).

(4) The foregoing embodiment has been described taking the inkjet printing apparatus 3 as an example of printing apparatus. This invention is not limited to the inkjet printing apparatus 3. For example, any printing mode will do as long as a printing apparatus performs printing while transporting an elongate printing medium.

(5) The foregoing embodiment has been described taking for example the transport path of the web paper WP constructed as shown in FIG. 1. This invention is not limited to such construction.

(6) The foregoing embodiment has been described taking the web paper WP as an example of printing medium and medium. This invention is not limited to such a printing medium or medium. This invention is applicable also to a printing medium and medium such as film, for example.

This invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention. 

What is claimed is:
 1. A transport control method for use in transporting a medium in a predetermined direction by an upstream drive roller disposed in an upstream position in a transport direction of the medium, and a downstream drive roller disposed in a downstream position in the transport direction, to operate the downstream drive roller by using PID control based on a detection value of a tension sensor disposed downstream in the predetermined direction of the upstream drive roller and upstream of the downstream drive roller for detecting tension of the medium, the transport control method comprising: a stable state determining step for determining, at time of the operation, whether a difference between the detection value of the tension sensor and a target value is within a stable width to make a stable state which maintains a stability time; and a gain decreasing step for decreasing a gain of the PID control to be less than an initial value when the difference is determined to be in the stable state.
 2. The transport control method according to claim 1, further comprising an initial value setting step for setting the gain to the initial value when the upstream drive roller starts to be driven from a stopped state, and when the upstream drive roller starts to be decelerated toward the stopped state.
 3. The transport control method according to claim 1, further comprising a gain increasing step for increasing the gain when, after the gain decreasing step, the difference is determined to have deviated from the stable width, or although within the stable width, is in a nonstable state incapable of maintaining the stability time.
 4. The transport control method according to claim 2, further comprising a gain increasing step for increasing the gain when, after the gain decreasing step, the difference is determined to have deviated from the stable width, or although within the stable width, is in a nonstable state incapable of maintaining the stability time.
 5. The transport control method according to claim 1, wherein the stable state determining step has the stable width in at least two types.
 6. The transport control method according to claim 1, wherein the gain decreasing step has a minimum gain set as a lower limit in decreasing the gain to be less than the initial value.
 7. The transport control method according to claim 1, wherein the gain increasing step has a maximum gain set as an upper limit in increasing the gain.
 8. The transport control method according to claim 2, wherein the gain increasing step has a maximum gain set as an upper limit in increasing the gain.
 9. The transport control method according to claim 1, wherein the gain is a proportional gain.
 10. A transport apparatus for transporting a medium in a predetermined direction, comprising: an upstream drive roller disposed in an upstream position in a transport direction of the medium; a downstream drive roller disposed in a downstream position in the transport direction; a tension sensor disposed downstream in the predetermined direction of the upstream drive roller, and upstream of the downstream drive roller, for detecting tension of the medium; a drive control unit for operating the downstream drive roller by using PID control based on a detection value of the tension sensor; a stable state determining unit for determining, at time of the operation, whether a difference between the detection value of the tension sensor and a target value is within a stable width to make a stable state which maintains a stability time; and a gain adjusting unit for decreasing a gain of the PID control to be less than an initial value when the stable state determining unit has determined that the difference is in the stable state.
 11. The transport apparatus according to claim 10, wherein the gain adjusting unit sets the gain to the initial value when the drive control unit starts to drive the upstream drive roller from a stopped state, and when the drive control unit starts to decelerate the upstream drive roller toward the stopped state.
 12. The transport apparatus according to claim 10, wherein the gain adjusting unit increases the gain when, after the gain adjusting unit decreased the gain, the stable state determining unit determines that the difference has deviated from the stable width, or although within the stable width, is in a nonstable state incapable of maintaining the stability time.
 13. The transport apparatus according to claim 11, wherein the gain adjusting unit increases the gain when, after the gain adjusting unit decreased the gain, the stable state determining unit determines that the difference has deviated from the stable width, or although within the stable width, is in a nonstable state incapable of maintaining the stability time.
 14. The transport apparatus according to claim 10, wherein the stable state determining unit has the stable width in at least two types.
 15. The transport apparatus according to claim 10, wherein the gain adjusting unit has a minimum gain set as a lower limit in decreasing the gain to be less than the initial value.
 16. The transport apparatus according to claim 10, wherein the gain adjusting unit has a maximum gain set as an upper limit in increasing the gain.
 17. The transport apparatus according to claim 11, wherein the gain adjusting unit has a maximum gain set as an upper limit in increasing the gain.
 18. The transport apparatus according to claim 10, wherein the gain is a proportional gain.
 19. A printing apparatus for performing printing while transporting an elongate printing medium in a predetermined direction, comprising: a printing station for printing on the printing medium in a printing area disposed along a transport path of the printing medium; an upstream drive roller disposed upstream of the printing area; a downstream drive roller disposed downstream of the printing area; a tension sensor disposed downstream in the predetermined direction of the upstream drive roller, and upstream of the printing area, for detecting tension of the printing medium; a drive control unit for operating the downstream drive roller based on a detection value of the tension sensor and by using PID control; a stable state determining unit for determining, at time of the operation, whether a difference between the detection value of the tension sensor and a target value is within a stable width to make a stable state which maintains a stability time; and a gain adjusting unit for decreasing a gain of the PID control to be less than an initial value when the stable state determining unit has determined that the difference is in the stable state.
 20. The printing apparatus according to claim 19, wherein the gain adjusting unit sets the gain to the initial value when the upstream drive roller starts to be driven from a stopped state, and when the upstream drive roller starts to be decelerated toward the stopped state. 